Electron energy analyzers are well known in the art. Such devices are used to analyze a beam of electrons having differing kinetic energies to provide information about the composition of materials from which the electrons were emitted, since different elements emit electrons with characteristic kinetic energies. In addition to spectral information, some electron energy analyzers also preserve information about the spatial distribution of the electrons in the incident beam, so that in addition to spectral information it is possible to determine where in the sample a particular element is located.
The illuminating source of electrons is generally a spatially dispersed, roughly parallel beam of electrons which are focused to an entrance aperture or slit of the analyzer, which defines the initial position of the electrons. The beam of electrons are then subjected to electric or magnetic fields, generally perpendicular to the incident beam, which cause electrons of differing kinetic energies to follow different trajectories. The fields in the analyzer cause all electrons of a given kinetic energy to be re-focused on an exit plane, hence causing the electrons to be spatially dispersed according to their kinetic energy. By placing an exit slit at an appropriate location in this plane it is possible to select electrons of a specific energy to exit, either for detection or further processing.
A common analyzer which has been widely used is a hemispherical analyzer, or spherical capacitor analyzer (SCA). In an SCA, the entrance and exit apertures are in the same plane, and the electrons follow orbits which are approximately 180.degree. segments of circles. By placing appropriate lenses before and after these two apertures, it is possible to reconstruct the initial spatial distribution of the electrons to provide spatial information about the energy of the analyzed electrons. In this way it is possible, for example, to determine where in a material being analyzed a particular element is located.
There are several problems with current electron analyzers which provide both spatial and spectral information. One problem is that the lenses placed before and after the hemispherical analyzer distort the image so that small features cannot be resolved. Also, the hemispherical analyzer has internal aberrations which degrade image quality. In addition, the combined system has very low throughput--that is, many of the incident electrons are lost and are not present in the final image, so that the sensitivity of the analysis is very low. It is very difficult to trade off spectral resolution for image quality.
As a result, a heretofore unsolved need exists for an electron energy analyzer that enables the user to selectively focus electrons without dispersion and with decreased spherical aberrations.